1. Field of the Invention
The present invention relates generally to a mounting system for mounting a structure whose thickness cannot be tightly controlled, and particularly to a mounting system for mounting an image sensor.
2. Background of the Prior Art
A typical image sensor chip 12 of the type mounted in various types of devices, such as medical instruments, video cameras, and bar code readers is shown in FIG. 6. The image sensor shown includes a bottom planar member 110 carrying a pixel array 112. Front and rear lead frames 114 initially extend peripherally from the pixel array and are formed to extend downwardly about front and rear edges respectively, of bottom planar member 110 terminating in pins 32. Image sensor 12 further includes top planar member 118 which rests against pixel array 112 and lead frame 114. Top planar member 118 is secured against lead frames 114 and against pixel plane 112 by the force of adhesive material interposed between top and bottom planar members 110 and 118. Adhesive material is disposed mainly about the periphery of pixel array 112. In addition, image sensor 12 may include a glass layer 120. In some popular models of image sensors, top planar member 118 is configured in the form of a frame which retains glass layer 120. Thus, it is seen that image sensor 12 is of a stacked-up configuration. Like most structures whose design is of a generally stacked up configuration, the thickness, t, of assembly 12 cannot be tightly controlled. In the manufacturing of sensor 12, the thickness of the various layers will vary from structure to structure. Accordingly, the total thickness, t, will vary from structure to structure. The spacing, s, between top and bottom planar members 110 and 118 of image sensor 12 is particularly difficult to control given that such spacing is a function of the amount of adhesive used, the thickness of pixel array 112 and the thickness of lead frames 114.
Particularly in applications where such an image sensor must be side mounted (not "plugged into" a PCB), as is the case with most bar code reader applications, then the inability to tightly control image sensor thickness, t, can negatively impact operational characteristics of the device in which the sensor is incorporated in. An explanation of how the inability to tightly control sensor thickness can impact operation of a bar code reader is made with reference to FIGS. 7 and 8 showing a multilayered image sensor incorporated in a bar code reader according to a prior art mounting scheme. In the mounting scheme shown, a multilayered image sensor 12 is disposed into a holding pocket 16 defined by substantially equally tensioned pairs of rear pins 19 and forward pins 18. The prior art mounting system may further include a spacer 21 for biasing sensor 12 forwardly against forward pins 18.
A number of operational problems can arise with this mounting scheme. If the thickness of the image sensor which is manufacturable to a thickness in the tolerance range from T.sub.min to T.sub.max tends toward T.sub.min then pins 18, 19 may not supply sufficient pressure to image sensor 12 to hold sensor 12 in a secure position. Further, it can be seen that the distance, d, from any fixed point in space, P.sub.s, to any fixed point P.sub.p, on the plane of pixel array 12 will vary depending on the total thickness, t, of sensor 12 which is a thickness having a high degree of variability. This is not preferred since controlling the distance, d, is important to controlling the operation of the reader.
There is a need for an image sensor mounting system for mounting an image sensor in an imaging device which minimizes operational problems resulting from the inability to tightly control an image sensor chip's thickness.